Glass manufacture involves the mixing of various batch ingredients, generally including silica sand, dry powders, granular oxides, carbonates, cullet (i.e., broken and/or recycled glass), and other raw materials (depending on the desired type of glass) and heating them to a temperature of about 1500.degree. C., wherein they become molten and acquire a homogeneous nature. In general, substantial quantities of heat are required for the melting process, this heat generally supplied by combustion of fossil fuels. Because of the relatively poor heat transfer from the hot flue gases to the pool of molten glass, exhaust gas temperatures from the process are usually quite high, in spite of various types of heat recovery equipment employed. Also, pollutants of various types are emitted from the melting process along with the exhausted flue gases.
Two areas of improvement to the basic glass manufacturing process are desirable, namely (1) better energy efficiency, which can be achieved by preheating batch materials using exhaust gas heat with corresponding reductions in fuel requirements, or alternatively, more glass can be made with the same energy input to the melting process; and (2) reduced pollution emissions, wherein various types of gas absorption and/or dust filtration systems can be implemented to satisfy government regulations. The prior art has long investigated improvements in these two areas, and as a result, improvements have been implemented in glass manufacturing facilities in various ways in both production and pilot plants.
The present invention relates to a novel means of achieving both of the above improvements in one system and to the novel arrangement which achieves a functioning system incorporating the improvements.
With respect to better energy efficiency, the glass industry has always been concerned with the energy efficiency of the glass melting process, and has routinely implemented equipment for preheating of combustion air with waste heat from exhaust gases. For over 35 years, interest has also existed for preheating of batch materials. Initial interest was directed more towards presintering the glass batch to promote certain chemical reactions between the glass making materials, as opposed to utilizing waste heat per se. The prior art includes a large variety of methods for heating glass batches, utilizing both direct and indirect flue gas contact and batches in raw powder or agglomerated form.
Preheating of glass batch is desirable for three major reasons:
(1) Improved overall thermal efficiency of the glass melting process utilizing waste heat from exhaust gases. About half of the theoretical energy needed to produce container glass from conventional glass-making raw materials is required to heat the raw materials up to 750.degree. C.
(2) Reduced volatilization and resulting pollutants as a consequence of lowering melting temperatures and prereaction of batch materials.
(3) Faster and more uniform melting, especially where agglomerated batch is utilized.
With respect to pollution capture, the nature and amounts of pollution emissions from glass melting furnaces vary considerably within the glass industry, depending upon the type of glass and production method used. Generally, pollutants fall into two general categories, particulate and gaseous. Particulate pollutants can be ash components in the fuel, carryover of batch material, r products of condensation of material volatilized from the glass melt. The latter is the most prevalent and the primary particulate from soda-lime glass furnaces is Na.sub.2 SO.sub.4 resulting from Na and SO.sub.2 volatilized from the glass melt. Particulate emissions from glass furnaces can be reduced somewhat by reducing temperature and as a result volatilization from the surface of the molten glass. The use of preheated glass batch permits a lowering of the furnace temperature and in itself decreases particulate emissions.
Particulate material from glass melting furnaces is extremely difficult to capture owing to its small size, typically 0.2-0.7 .mu.m. Generally, electrostatic forces are required to capture particles of such small size. In fact, electrostatic precipitators have become the glass industry standard for capture of particulate matter.
Gaseous emissions from soda-lime furnaces include sulfur and nitrogen oxides, with sulfur oxides resulting primarily from sulfur components in the batch material and nitrogen oxides resulting from oxidation of N.sub.2 contained in combustion air. Conventional technology for reduction of SO.sub.2 emissions are lime based wet scrubbers. Both these and electrostatic precipitators are add-on devices to the glass manufacturing process which carry significant penalties to the production economics.
Conventional equipment for nitrogen oxide emission reduction has not yet found widespread use. A lowering of furnace temperature should result in reduced nitrogen oxide emissions, so batch preheating would have a beneficial effect here also.
Batch preheating combined with pollution reduction is disclosed in U.S. Pat. No. 4,338,113, relating to a direct/indirect heat exchanger, wherein hot flue gases are directly contacted with durable granular material (such as gravel) in a filter bed. Heated granules are transported to a mixing drum where they are contacted with batch materials, thereby heating the batch materials and cooling the granules. Cooled granules are returned to the filter bed.
The prior art has recognized the potential for simultaneous pollution reduction with batch preheating, but not only from source reduction, as mentioned above. Generally, it has been suggested to use batch preheating in schemes where exhaust flue gases are brought into direct contact with batch materials. Then the batch, whether in raw, loose form or agglomerated form, is expected to function as a mechanical collection site for particulate pollutants. Also, certain components of the glass batch (typically soda ash for soda-lime glass) are chemically reactive with gaseous phase pollutants (notably SO.sub.2 for soda-lime glass) and the gas solid reaction can effectively remove the pollution. While SO.sub.2 reductions have been easily achieved, actual attempts at simultaneously preheating a glass batch and reducing particulate pollution have typically failed.
Hence, there remains a need in the art for a workable arrangement for both preheating a glass batch and simultaneously reducing particulate pollution.